Ophthalmic lenses with light sources and dioptricity of 60 D and higher are known as Slit lamp lenses. Ophthalmic lenses with light sources and dioptricity of 40 D and lower are known as BIO lenses. They are used for indirect ophthalmoscopy with slit lamps or a light attached to a headband. In this case, a light source is a headlight, which is secured on the ophthalmologist's head, or a lamp that directs the light directly into the patient's eye via special mirror. The instruments that are used in the latter case are known as ophthalmoscopes.
Ophthalmoscopy is a test intended for observation of the fundus of the eye and other structures of the posterior chamber of an eye. The ophthalmoscopy is an eye examination procedure performed by an oculist as a part of a routine physical examination. It is important for determining the health of the retina, optic disc, vitreous humor, etc.
Prior Art section of U.S. Pat. No. 4,627,694 issued on December 1986 to David Volk describes the history of the ophthalmoscopy.
Single-piece lenses for observation of the posterior part of the vitreous cavity and the fundus of an eye were introduced into the ophthalmic practice in the 1950th.
First, conventional optical lenses with plano and spherical surfaces of various dioptric powers were used. Then, in 1956 and again in 1957 David Volk introduced so called conoid ophthalmic lenses. These conoid lenses were composed of one conoid surface and one spherical or plano surface, and also composed of two conoid surfaces. Later, such conoid ophthalmic lenses were used under the terms “aspherical lenses”, “lenses with aspheric surfaces” generally accepted in optics. In the field of the ophthalmoscopy such lenses became known as Biolenses and Slit Lamp lenses.
An ophthalmic lenses of the type mentioned above differs from conventional optical lenses in that they are used in optical systems wherein an entrance pupil of the patient's eye plays a role of an optical lens and a diaphragm of a variable diameter.
Further development of ophthalmic lenses is presented in the number of patents granted to David Volk and Donald Volk.
In particular, U.S. Pat. No. 4,738,521 issued on Apr. 19, 1988 discloses a lens for use in indirect ophthalmoscopy. The lens has two functions: firstly, as a condensing lens converging light from an ophthalmoscope light source to the pupil of the eye and thereby illuminating the fundus of the eye, and secondly, as an image forming lens which forms an aerial image of the fundus of the eye, which is viewed monocularly with a monocular indirect ophthalmoscope or binocularly and stereoscopically with a binocular indirect ophthalmoscope. Both the front and back surfaces of the lens are positive aspheric surfaces of revolution of an aspheric type on a common axis of revolution, the dioptric power at the apex of the front surface of the lens being approximately twice that of the apex of the back surface of the lens, wherein the back surface faces the eye being examined. The eccentricity of the front surface has a definite relationship to the eccentricity of the back surface, the eccentricities of the two surfaces of the lens being a function of the sum of the dioptric powers of the two surfaces of the lens. The eccentricities and apical dioptric powers of the surfaces of the lens are such that the lens converges the light from the ophthalmoscope light source to a precise image of the source at the entrance pupil of the eye, and simultaneously the lens forms, with the light emerging from the eye, a substantially flat aerial image of the fundus of the eye in which images the aberrations of the image including curvature, astigmatism and distortion are optimally corrected.
U.S. Pat. No. 5,046,836 issued on Sep. 10, 1991 relates to a diagnostic indirect ophthalmoscopy contact lens system. A compound diagnostic indirect ophthalmoscopy contact lens described in this patent is intended for illumination and observation of the fundus of the eye. This two-lens assembly contains a plus powered meniscus aspheric contact element and a biconvex aspheric anterior element, each of the lens elements contributing positive refractive power to the optical system and co-acting to illuminate and form an aerial image of the fundus of the eye. For observation of the eye fundus, the device is brought into contact with the eye cornea. Such devices did not find wide application because of their relatively complicated use.
U.S. Pat. No. 5,333,017 issued on Jul. 26, 1996 discloses a lens specifically designed for use with a slit lamp biomicroscope in the examination of a patient's eye. One or more lens elements having first and second convex aspheric surfaces of revolution may be used. The first and second aspheric surfaces are coaxial and non-symmetrical with respect to one another. The aspheric surfaces are chosen to correct astigmatic imagery of the lens, with the formed aerial image free of excessive field curvature and astigmatism. The lens is held at a distance from the patient's eye pupil corresponding to the secondary focal length of the lens. If the examined eye is emmetropic and the lens is held in a position wherein the entrance pupil of the lens is conjugate with that of the examined eye, an image of the entrance pupil of the patient's eye will be formed at the pupil aperture of the optical system of the slit lamp biomicroscope used to observe the aerial image of the fundus as produced by the lens. The ratio of the apical radius of curvature of each surface and the ratio of the apical eccentricities of each surface are chosen to optimally correct for astigmatic imagery as well as the pupil imagery of the lens, being dependent upon the index of refraction of the optical quality glass or plastic used in the production of such lenses. The indirect ophthalmoscopy lens of the invention therefore provides a sharper, focused fundus image and a relatively wide field of view.